U.S. Environmental Protection Agency Administrator Scott Pruitt issued a Notice of Proposed Rulemaking on Tuesday that aims to repeal the Obama administration’s “Clean Power Plan,” fulfilling a campaign promise made by President Trump.

What is the Clean Power Plan?

The plan was an Obama administration policy designed to reduce carbon dioxide emissions from electrical power generation by 32 percent by 2030, compared with 2005 levels. The plan set individual state targets for greenhouse-gas reductions and required states to submit plans on how they would meet the targets.

Gene J. Puskar

EPA Administrator Scott Pruitt speaks to a group of coal miners in Sycamore, Pa., in April.

Did every state have to reduce emissions by the same amount?

No. Some states with more carbon-intensive energy sources — coal-burning power plants — had to reduce emissions by significant amounts, while other states were on track to meet the targets without intervening. According to the Rhodium Group, 21 states would have had to do more to comply with the plan, including Pennsylvania and New Jersey.

Did the plan go into effect?

No. Challengers sued, arguing the EPA overstepped its legal authority in issuing the plan. The U.S. Supreme Court in 2016 ordered the EPA to halt enforcement of the plan until a lower court rules in the lawsuit. That case is still pending before the U.S. Court of Appeals for the District of Columbia.

Does Pruitt’s action end the matter?

No. Supporters of the Clean Power Plan promised to challenge Trump’s action, saying the government has a legal obligation to address climate change after the Supreme Court in 2007 held that greenhouse gases are pollutants under the Clean Air Act. In 2009, the EPA determined that greenhouse-gas emissions endanger the public health and welfare of current and future generations. Those findings are the legal foundation for the Clean Power Plan.

Why does Pruitt argue the Clean Power Plan is illegal?

EPA regulations typically set performance standards for emissions sources like power plants based upon best available technology. The Clean Power Plan “is formulated in reliance on and anticipation of actions taken across the electric grid, rather than actions taken at and applied to individual units,” according to the EPA’s repeal proposal. It sets standards that are impossible for individual plants to meet.

Did the administration propose an alternative?

No. Pruitt said the first action was to clean the regulatory slate. “We can now assess whether further regulatory action is warranted; and, if so, what is the most appropriate path forward, consistent with the Clean Air Act and principles of cooperative federalism,” he said.

What are the costs of a Clean Power Plan repeal?

The Trump administration estimates the proposed repeal could provide up to $33 billion in avoided compliance costs in 2030. It also dramatically discounted the Obama administration’s estimates of the plan’s health and environmental benefits. The EPA’s recalculations employ “math tricks” that will likely be challenged in court, according to David Doniger, a Natural Resources Defense Council climate expert.

What happens now?

Pruitt’s action starts a rule-making process that will allow the public 60 days to comment. Any final EPA rule will face court review.

Does the abandonment of the Clean Power Plan mean there will be a return to old ways of producing electrical power?

Larger economic forces are already shifting the nation’s energy mix. Emissions dropped from 2005 levels because of the shift to high-efficiency gas power plants and wind and solar energy. In addition, several large states have pledged to move forward with their own carbon-reduction goals.

Will the plan’s repeal save the coal industry?

It’s doubtful. It may slow the transition to cleaner energy sources by delaying the premature retirements of coal-fired power plants. And in regions that have fuel-switching capacity, coal may experience a short-term revival if natural gas prices increase and coal becomes more competitive. But with coal’s uncertain future, investors are unlikely to back the construction of new coal-fueled power plants when less risky alternatives are available.